The present invention relates to an optical detector which is effective for detecting phase modulated signals in a light beam, but which is relatively insensitive to noise induced when the light beam traverses an aberrant path, caused by (i) a turbulent atmosphere, (ii) relative platform motion or (iii) other artifact which induces a relatively low frequency aberration of the signal. The aberrations can range from near DC to hundred""s of kilohertz while the desired phase encoded data or other information may fall into the multi-GHz range. The present invention may be used in a number of applications and methods, including machines and methods for the testing of materials, communication systems and methods, etc.
The optical detector of the present invention can be used in laser communication, remote sensing, and nondestructive testing applications. Remote sensing and high-bandwidth (multi-GHz) optical communication receivers have a need for an optical detector which can operate under conditions of extreme beam wander, static and dynamic optical distortions (turbulence, speckle, modal dispersion in multi-mode fibers), and/or relative platform motion. A dual-fiber system can be used for applications where secure links are required, with low probability of interception and detection.
In the manufacturing arena, there is a need for optical testing and process control of critical components. Laser-based ultrasound (LBU) represents a viable approach to satisfy these needs, and while prior art photo-emf sensors can form an important component in such diagnostic systems, the present invention will enable such sensors to perform with even better performance. LBU systems can significantly decrease the inspection time and increase reliability, and when the present invention is utilized, it can operate in adverse factory environments, in addition to a strictly controlled laboratory environment. This is due to the fact that the present invention can compensate for in-factory vibrations, relative platform motion, and/or variations in the temperatures and concentration of particles.
The prior art includes single-mode optical fiber delay lines, which are suitable for single-spatial mode optical channels (e.g., fiber networks, etc.) Such prior art systems do not function with any reasonable efficiency, particularly in the case of multi-mode beams.
In terms of multi-spacial mode systems, the prior art includes Fabry Perot interferometers, which are bulky, expensive (especially if one has large field of view needs), and require servo-control for optical phase-biasing.
The prior art includes U.S. Pat. No. 5,900,935 to Klein et al., which discloses a homodyne interferometer. In this prior art system, an optical beam is directed via two paths to a holographic element. One of the paths includes a sample, off which the optical beam is reflected. The optical lengths of the two paths must be kept less than the coherence length of the laser used to illuminate the sample. This restriction imposes strict limitations on the distance from the testing apparatus to the sample.
The present invention replaces the bulky prior art interferometer with a compact, multi-mode fiber delay line, integrated with a real-time wavefront matching element, which is automatically biased (so no servo control system is required) for maximum quadrature detection.
The prior art also includes isolated two-wave mixers as well as isolated double-pumped phase-conjugate mirrors, both used as real-time beam clean-up (or wavefront matching) elements; these systems degrade in performance in the case of large-amplitude phase excursions, since the real-time grating can experience erasure.
The prior art further includes the integration of a multi-mode optical fiber time-delay line 125 with a photo-emf sensor 139 (see FIG. 1 and U.S. Pat. No. 5,684,592). This compact system can also coherently detect highly aberrated, multi-spatial-mode beams. But, the bandwidth of the photo-emf sensor is limited to about 100 MHz, which limits the system""s utility for use in many communication systems, where multi-GHz bandwidth channel capacity may be well required. Moreover, the detection sensitivity is electronic-noise limited and is about an order of magnitude less sensitive than the shot-noise limit.
The present invention overcomes all theses limitations, by integrating a high-performance adaptive optical combined element, with a multi-spatial-mode fiber delay line. Moreover, by using a pair of such delay lines, a short-coherence length source can be used; the prior art in this respect involves a photo-emf sensor, which is integrated with the dual-fiber delay line (see FIG. 3). Therefore, the net system is limited in detection bandwidth. Finally, the multi-mode optical fiber delay line can be in the form of an amplifying multi-mode optical fiber (e.g., Er-doped glass), for added gain. The present invention will provide robust sensors which can perform in a variety of adverse industrial conditions, including the use of short-coherence sources, extreme (i.e., many optical wavelengths of) workpiece wobbling and beam wander, low-reflectivity workpieces (e.g., or other propagation path losses), and laser amplitude fluctuations (due to workpiece reflectivity changes, wobbling, etc.). The present invention also provides robust sensors for remote sensing and laser communications applications in which the sensor must tolerate fluctuations in received intensity levels.xe2x80x9d
Briefly, and in general terms, the present invention provides an optical apparatus for coherent detection of an input optical beam. The apparatus includes a beam splitter for splitting the input optical beam into a first component and a second component; an optical delay device arranged to receive the second component, the optical delay device imposing an intentional delay in the second component of the input optical beam; and an adaptive beam combiner coupled to receive the second component with a delay imposed thereon by the optical delay device; and the first component from the beam splitter. The adaptive beam combiner has two exiting components which have the same wavefronts and propagating directions as the first and second components and which are in quadrature. A detector arrangement is provided for receiving and detecting the first and second exiting components from the adaptive beam combiner.